I'm surprised no-one's mentioned
Midsommar yet. I don't think it's anywhere near as good as many people think it is (and it's not a patch on The Wicker Man to which it's frequently compared) but it's still a worthy entry in the canon. Florence Pugh is once again on top form even if I don't think the rest of the film came together as well as it should have done.
Ken Russell's
The Lair of the White Worm, loosely based on the Lambton Worm, is a very Russell take on the tale - lots of black comedy for those who can tolerate his style.
I'm not sure if
The Witchfinder General counts, but it does in my book - albeit with witchcraft being the backdrop to the real horror.
Super-obscure, but there's a 60s portmanteau film from Japan called
Kwaidan, IIRC all based on traditional Japanese folklore and presented in a very theatrical style - well worth seeking out.
Speaking of which, is it old enough now to consider
Ring as folk horror of the 90s? Watched this again recently (Eureka did a nice blu-ray of it) and still a stone-cold classic.
Other than the three mentioned in the OP, I would add The VVitch as the best the genre has to offer.
Robert Eggers' other film,
The Lighthouse, also counts as a folk horror IMHO and is, in my opinion, an even more stonking film than The Witch.
Of course, an evening of folk horror wouldn't be complete without a showing of
Troll 2, a horror in every sense of the word.
